TERTIARY. 425 



existing types ; every stage brings us into contact with forms 

 nearer to those now living. It was for this reason that Lyell in 

 1830, in conjunction with G. P. Deshayes, proposed the terms 

 Eocene, Miocene, and Pliocene for the three great divisions into 

 which European Tertiary deposits had been divided. These terms 

 were based upon the percentage of recent Mollusca found in the 

 strata to which they were applied.' Thus the Eocene strata (dawn 

 of recent) contain a very small proportion of living species ; the 

 Miocene strata (less recent), although containing more recent 

 species, yet contain a minor proportion compared with the 

 Pliocene strata (more recent), which contain a plurality of recent 

 species. These names are now used simply as stratigraphical 

 terms, for Lyell himself subsequently admitted that the Eocene, 

 Miocene and Pliocene periods have been made to comprehend 

 certain sets of strata, of which the fossils do not always conform 

 strictly, in the proportion of recent to extinct species, with the 

 definitions first given by him, or which are implied in the 

 etymology of those terms. The name Oligocene (signifying 

 little recent, or few forms of life recent) was proposed in 1854 by 

 Prof. H. E. Beyrich, for the strata known as Upper Eocene, 

 because in their fauna and development he considered the beds 

 entitled to distinct recognition. The term has been very largely 

 adopted. 



From astronomical calculations based on the considerations of 

 climate, it has been calculated that the Eocene period occurred 

 from 2,620,000 to 2,460,000 years ago.^ 



The Tertiary strata exhibit marine, estuarine, freshwater and 

 terrestrial conditions, repeatedly changing ; some of the divisions, 

 and especially the Middle Eocene (termed Nummulitic by 

 D'Archiac in 1842), were spread more or less continuously over 

 a wide area of the earth's surface.'' 



The Tertiary group is subdivided as follows : — 



Pliocene. 



Miocene (wanting in England'and Wales'*). 



Upper (Oligocene). 

 Eocene^ I Middle. 



Lower. 



The Eocene strata now occupy two distinct tracts, termed 

 respectively the London and Hampshire (or Isle of Wight) Basins. 

 The synclinal or basin-like arrangement of the Chalk strata upon 

 which these newer deposits lie is, perhaps, more conspicuous 



^ Principles of Geology, edit. i. ; see also Elements Geol. ed. vi. p. 187 ; and 

 Students' Elements, 1871, p. 121. The names are sometimes spelt Meiocene and 

 Pleiocene. " Croll, Climate and Time, 1S75. 



^ See J. S. Gardner, P. Geol. Assoc, vi. 90. 



* The Bovey Beds were formerly grouped as Miocene, and the Hempstead Beds 

 have also been classed with that system. 



^ The term Hantonian was proposed by Mr. Jukes-Browne for the Eocene and 

 Oligocene strata, G. Mag. 1885, p. 293. 



